The present invention relates to laboratory calibration of electrical transducers of the type employed for providing an electrical signal indicative of moment or torque load applied to a shaft. Such transducers are widely employed for in-service indication of torque loads applied to shafts in machinery, drive mechanisms, and machine tools. Typically, such transducers are of the type employing electrical resistance strain gauges bonded to a strain sensitive member connected between the torque input and output members of the transducer.
Transducers of the aforesaid type are often required to maintain an accuracy of a one tenth of one percent (0.10%) and in some application one-twentieth of a percent 0.05% of full-scale reading for the range of torque loads the transducer is intended to measure. It will be seen that calibration of such a transducer against known or reference torque loads must be extremely precise in order to determine if the transducer is capable of the desired accuracy over its measurement range.
The aforesaid type torque transducers are employed in some applications to measure very high torque loadings in the order of 300,000 inch-pounds (33,900 newton-meters). However, it has been found quite difficult to apply such high torque in the laboratory with the necessary degree of precision required for calibrating a torque transducer.
This difficulty will be better appreciated by way of example in which a moment arm beam having a length of 120 inches (305 cm) is loaded at the end thereof by precision weights which are applied in increments ranging from 0 to 1,000 pounds (454.5 kilograms). In order to obtain the desired accuracy of calibration of the transducer, the 120 inch moment arm of the loading beam at the point of load attachment must not vary during calibration testing by more than 1 milli-inch (0.0254 millimeters).
Where pinned or knife-edge weight hangers are employed for the loading of the moment arm, or beam, it will be seen that a slight amount of rotation of the horizontally disposed moment beam, with the weight hangers maintaining a vertical orientation under gravitational forces, causes a shortening of the effective moment arm. The amount of shortening is equal to the length of the moment arm, to the point of weight attachment, multiplied by the sine of the angle of rotation. Alternatively, the effective moment arm is equal to the moment beam length, at point of weight hangar attachment, multiplied by the cosine of the angle of beam rotation.
Shortening of the effective moment arm by only a small angle of rotation can thus produce an error in the applied moment or torque by an amount which exceeds the permissible deviation and results in inaccurate loading of the transducer during calibration. Heretofore, with pinned or knife-edge weight hangars, in order to eliminate this shortening of the moment arm, it has been necessary to level the moment arm after the application of each increment of deadweight load by rotation of the reaction member attached to the opposite end of the loaded transducer to accommodate the rotational deflection of the transducer. This has resulted in a time-consuming and thus costly procedure for calibrating torque transducers for use in measuring high torque loadings.
Thus it has long been desired to find a way or means of precisely calibrating an electrical torque transducer in the laboratory where very high torque loading is required, and to provide such calibration by incremental loading without the necessity of "zeroing" or re-leveling the calibrating beam with each increment of load.